Same Melt-and-Freeze Trick, Utterly Different Rules
At heart, injection molding and extrusion are casting-for-plastics: melt a material, force it into the shape you want, and let it freeze. So everything you banked in the casting guide still applies — the processing-structure-property chain, the mold-wall chill, the residual stress, the honest fact that how it freezes decides how good the part is. But polymers break casting's rules in three ways that change everything. Instead of small round atoms that snap into a crystal, a polymer is a bowl of spaghetti: a tangle of macromolecules, each one thousands of atoms long, that can never fully line up. That single difference sets the whole chapter.
The first fork in the road is thermoplastic versus thermoset. In a thermoplastic the long chains are held to each other only by weak van der Waals attraction; heat jiggles them loose so the whole tangle can flow, and cooling lets it re-freeze — like candle wax, you can melt and re-shape it endlessly (polyethylene, polypropylene, PET, nylon). A thermoset is the opposite: heat and a hardener stitch the chains into one giant covalently crosslinked net, and once that net is set it can never melt again — like a fried egg you cannot un-cook, or a car tyre after vulcanization. This single fact decides which process you may even use, because extrusion and injection molding both need a material that can be re-melted, so they are almost entirely a thermoplastic game.
The second rule-breaker is that a polymer melt does not have one sharp melting point like a metal — it softens over a range, and it flows as a viscoelastic goo that is part honey (it flows) and part spring (it stores and springs back). The temperatures that matter are the glass transition temperature Tg, below which the frozen chains make the plastic hard and glassy, and above which they wriggle and it turns rubbery, and — for the ordered ones — the melting temperature Tm where any crystalline regions finally give way. Polystyrene has a Tg near 100 degrees C (glassy and brittle at room temperature, which is why a CD case snaps); natural rubber has a Tg near -70 degrees C (rubbery at room temperature, which is why it stretches). Processing means heating the plastic across these lines and freezing it back — and, crucially, remembering that the spring never fully lets go.
Extrusion: A Screw That Never Stops
Extrusion is the continuous end of the business. Pour solid pellets into a hopper; a big turning screw inside a heated barrel bites them, drags them forward, and squeezes them into a hot viscous melt; then it rams that melt through a shaped die and out comes an endless profile of whatever cross-section the die was cut to — pipe, sheet, cling film, the insulation sheath around a wire, a window frame, even spaghetti. It is the plastics cousin of the metal extrusion you met in the last guide, but with a telling difference: metal extrusion squeezes a solid billet that flows plastically, while here the polymer is a genuine melt, poured through the die like thick toothpaste and cooled only after it leaves.
The screw is the whole machine's cleverness. It is cut in three zones — a feed zone that bites the cold pellets, a compression zone whose channel gets shallower to squeeze out trapped air and finish the melting (much of the heat is not from the barrel heaters but from the sheer friction of the melt being worked, a compression ratio of about 3 to 1), and a metering zone that evens out the pressure and doses a steady flow to the die. Now the fingerprint appears: as the melt is dragged along the screw and stretched through the die, the long chains get combed out and aligned along the flow direction. That built-in molecular orientation makes the extrudate anisotropic — stronger along its length than across it, like wood, or like the grain in the last guide's rolled sheet. Fiber spinning pushes this to the limit, drawing the melt down so hard that the chains line up almost perfectly and the fiber becomes astonishingly strong along its axis.
A SINGLE-SCREW EXTRUDER (side view, schematic)
hopper (solid pellets)
| v v v
___|________________________________________________
| FEED | COMPRESSION | METERING | DIE ===> endless
|>>>>>>>>>>|>>>>>>>>>>>>>>>>|>>>>>>>>>>>>| (shape) ===> profile:
|__(((((((_|__((((((________|_((((_______|_________ pipe, sheet,
screw flights turn: drag + squeeze the melt -> film, fiber
[ barrel heaters + shear-heating from working ]
feed : bite cold pellets, convey them forward
compression : channel shallows -> squeeze out air, melt fully
metering : even out the pressure, dose a steady flow
die : sets the cross-section; melt is DRAWN + cooled AFTER
Chains get combed along the flow -> orientation -> anisotropy
(strong along the length, weaker across it).Injection Molding: Plastic's Die Casting
Injection molding is to plastics exactly what die casting was to metals: ram a hot melt at high pressure into a cold, closed, reusable steel mold, let it freeze to shape, then pop it open and eject the part. The machine is an extruder with a twist — the screw both melts the plastic and then slides forward like a plunger to shoot a measured shot into the mold cavity. A cycle takes only 15 to 60 seconds, and it repeats forever, which is why almost every small complex plastic thing you own — bottle caps, LEGO bricks, phone housings, syringe barrels — was injection molded by the billion. The mold, cut from tool steel, is the expensive part, so the process only pays off at high volume, exactly like die casting.
- Plasticize: the turning screw melts fresh pellets and gathers a measured shot of melt in front of its tip.
- Inject: the screw drives forward as a plunger, filling the cold mold cavity in a fraction of a second.
- Pack and hold: extra melt is pressed in to feed the shrinkage as the plastic freezes — the same anti-shrink battle as a casting's riser.
- Cool: hold the part in the cooled mold until it is rigid enough to hold its shape — usually the longest step of the cycle.
- Eject: open the mold, push the finished part out with ejector pins, and start over.
Now watch the fingerprint form, and notice it is the very picture from the casting guide. The melt that touches the cold steel wall freezes in an instant, trapping the chains mid-flow: it becomes a highly oriented, nearly amorphous skin — the plastic version of a die casting's fine chill zone. The core cools far more slowly, giving the chains time to fold into orderly stacks called spherulites, so the crystallinity there is higher. The result is a layered skin-core microstructure that is anisotropic and non-uniform through the wall — and because a semicrystalline plastic packs denser when it crystallizes, the part shrinks noticeably as it sets, typically 1.5 to 3 percent for something like polypropylene versus about half a percent for a glassy amorphous plastic.
Blow Molding and Thermoforming: Making It Hollow
How do you mold something hollow, like a bottle, when a solid mold cannot reach inside? Blow molding borrows the glassblower's trick you met in the ceramics-and-glass forming recap: form a soft tube of melt (called a parison), close a split mold around it, and inflate it with air until it balloons out and freezes against the mold's inner walls. That is how nearly every plastic bottle, jerry can, and fuel tank is made — a thin, uniform, seamless hollow shell in seconds.
The clever version turns orientation from a nuisance into the whole point. A clear PET drink bottle is stretch-blow molded: a thick test-tube-shaped preform is stretched lengthwise by a rod and blown outward at the same time, typically around 2 times axially and 4 times around the circumference. That biaxial stretch combs the chains flat in two directions at once, so the wall is strong and stiff in-plane rather than weak across one axis, and — the bonus — the stretching triggers strain-induced crystallization that leaves the bottle both clear and far tougher, plus a better gas barrier so the fizz stays in. Orientation, which you had to fight in a molded part, is here dialed up on purpose to do the work.
The gentlest family is thermoforming: take a flat sheet of plastic, heat it just above its glass transition until it goes soft and rubbery (not fully melted, just floppy), then drape or vacuum it down over a simple mold and let it cool stiff. Yogurt cups, blister packs, disposable trays, and car door panels all come this way. The tooling is cheap and huge parts are easy, but there is an honest catch: as the sheet is stretched over the mold it thins out most where it has stretched most, so a deep-drawn corner ends up the thinnest, weakest spot of the part — the price of forming a solid-ish sheet instead of pouring a melt.
The Fingerprint, and What You Cannot Undo
Step back and the polymer fingerprint reads like a shorter version of the metal one. Cooling rate sets crystallinity: quench fast and you freeze a clear, amorphous, floppier part; cool slowly and spherulites grow, giving an opaque, denser, stiffer, but slightly more brittle part — the same speed-versus-structure dial as a casting, just written in folded chains instead of grains. Flow sets molecular orientation, hence anisotropy. Uneven cooling sets residual stress and warping. And every joining of two flow fronts sets a weld line. Change the process and you change the fingerprint — the recurring theme of this entire rung.
The deepest honest line in polymer processing is about what you can take back. Because a thermoplastic's chains are only held by weak van der Waals bonds, you can melt the part down and mold it again — that is exactly why a PET bottle can be recycled and why a bad injection shot can be reground and re-run. A thermoset or a vulcanized rubber cannot: its permanent covalent crosslinks are the un-cooking-an-egg problem, so a cured epoxy circuit board or an old car tyre cannot be melted back, only shredded or burned. And remember the viscoelastic catch throughout — a plastic's stiffness and strength are not fixed numbers but depend on temperature and how long the load is held, so a part that passes a quick test can still creep and craze over years in the sun.
One thread now points straight at the final guide. Polymer 3D printing — the most common kind of additive manufacturing — is really injection molding's melt-and-freeze done one thin bead at a time, so it inherits every quirk in this guide (orientation along each bead, uneven cooling, residual stress) plus a brand-new one: the bonds between printed layers never fully re-melt into each other, so a printed part is notoriously weak in the build direction, its own kind of built-in weld line. Same physics you now understand, one more fingerprint to read — which is exactly where the last guide in this rung will take you.